1. Technical Field
The disclosure relates to a suspension system for an engine.
2. Description of Related Art
A suspension system for an engine in a vehicle includes an engine mount that supports the engine on the vehicle. The engine mount has the function of curbing transmission of engine vibration to the vehicle side, and damping vibration transmitted from a road surface to the vehicle. In the engine mount, the capability of curbing transmission of engine vibration to the vehicle side is enhanced as the spring constant of the engine mount is smaller. On the other hand, in the engine mount, the capability of damping vibration transmitted from the road surface to the vehicle is enhanced as the damping coefficient of the engine mount is larger.
In Japanese Patent Application Publication No. 2011-80492 (JP 2011-80492 A), the relationship between the spring constant and the damping coefficient of an engine mount when the engine mount is tuned to predetermined spring characteristics is disclosed. More specifically, the relationship between changes in the spring constant and changes in the damping coefficient with increase of the frequency of engine vibration, under a condition where the engine mount is tuned to the predetermined spring characteristics, is disclosed. In the engine mount tuned to the predetermined spring characteristics, the frequency at which the spring constant is maximized and the frequency at which the damping coefficient is maximized are close to each other. Further, if the spring characteristics of the engine mount are changed, the frequency at which the spring constant is maximized and the frequency at which the damping coefficient is maximized are kept close to each other, but the trend in change of the spring constant and the trend in change of the damping coefficient relative to the frequency shift or change with the frequency in synchronization with each other.
FIG. 5 shows an example of change of the spring constant k and change of the damping coefficient c with change in the frequency of engine vibration when the engine mount is tuned to given spring characteristics (which will be called “first spring characteristics”). The frequency of engine vibration increases as the engine speed increases. When the engine mount is tuned to the first spring characteristics, the frequency at which the spring constant k is maximized and the frequency at which the damping coefficient c is maximized are close to each other, and these frequencies are relatively large values.
FIG. 6 shows an example of change of the spring constant k and change of the damping coefficient c with change in the frequency of engine vibration when the engine mount is tuned to given spring characteristics (which will be called “second spring characteristics”) that are different from the first spring characteristics. When the engine mount is tuned to the second spring characteristics, the frequency at which the spring constant k is maximized and the frequency at which the damping coefficient c is maximized are close to each other, and these frequencies are relatively small values.
In the meantime, an occupant of the vehicle may feel uncomfortable when engine vibration is transmitted to the vehicle side. Accordingly, in order to curb transmission of engine vibration to the vehicle side as much as possible, it is desirable to enhance the capability of curbing transmission of engine vibration to the vehicle side in the engine mount over the entire change region of the engine speed, in other words, to reduce the spring constant k of the engine mount over the entire change region of the engine speed.
To achieve the above condition, it may be considered to employ a technology as disclosed in Japanese Patent Application Publication No. 5-139165 (JP 5-139165 A) for switching spring characteristics of the engine mount according to the engine speed, and switch the spring characteristics in the following manner. Namely, the spring characteristics of the engine mount are switched to the first spring characteristics (FIG. 5) when the engine speed is low (e.g., lower than 1200 rpm), whereas the spring characteristics of the engine mount are switched to the second spring characteristics (FIG. 6) when the engine speed is high (e.g., equal to or higher than 1200 rpm).
By switching the spring characteristics of the engine mount according to the engine speed, it is possible to reduce the spring constant k of the engine mount over the entire change region of the engine speed, and enhance the capability of curbing transmission of engine vibration to the vehicle side in the engine mount. Accordingly, engine vibration is less likely or unlikely to be transmitted to the vehicle side, over the entire change region of the engine speed, and the occupant of the vehicle is less likely or unlikely to feel uncomfortable due to the transmission of the vibration.